2 Answers

This is the best combo I found for Bosses so far. First you lower their Defenses. Then you inflict a status(Preferably one that constantly lowers health). Then you Attack them with their lowered Defenses and get off that(as Shofu would say it)MAJOR DAMAGE. Finally you increase your own stats(Preferably Attack) to get off even more damage the next turn. Healing move as final if you just want healing from non-items thought I dont see why you would.

Boss Suicide
Sunny Day is in action then have only one Pokemon use Memento+Synthesis now the boss Pokemon is doing -1 or -2 damage do nothing and have a laugh while you see the Pokemon struggle to beat you.